Infrared laser light, which has a high energy output and is absorbed much into water, is useful for industrial laser processing devices and also for medical laser scalpels. Especially, devices using infrared laser light such as Er-YAG laser light oscillating at a wavelength of 2.94 μm, CO2 laser light oscillating at a wavelength of 10.6 μm, or the like are used as products in medical and industrial fields.
In the industrial fields, such infrared laser light is used for cutting various materials, welding, and surface modification. In the medical field, such infrared laser light is used for the purpose of incision, coagulation, transpiration, hemostasis and the like of biological tissues. Infrared laser light used in these fields has a significantly high energy output than laser light used in information and communication fields.
Infrared laser light used in the medical and industrial fields has a high energy output as described above, and is invisible. Therefore, a waveguide path of such infrared laser light needs to be monitored in real time regarding soundness to prevent breakage or any other flaw, and also needs to be sufficiently safe even when any flaw occurs.
For example, in order to prevent infrared laser light, which is invisible laser light, from illuminating an unintended position or in order to monitor the soundness of the waveguide path, infrared laser light and visible light are superimposed on each other. In this way, the position illuminated with the infrared laser light is specified and thus the infrared laser light is prevented from illuminating an unintended position.
Quartz-based optical fibers, which are widely used for information and communication purposes and illumination purposes, cannot be used as a waveguide path for infrared laser light having a wavelength of 2 μm or longer, such as Er-YAG laser light or CO2 laser light mentioned above. Therefore, fluoride-based optical fibers, silver halide-based optical fibers, chalcogenide-based optical fibers and the like, which are formed of a material transmissive of light in an infrared light wavelength region, are used.
However, the fluoride-based optical fibers can merely guide infrared laser light having a wavelength of about 3 to 4 μm at the longest and thus cannot be used as a waveguide path for CO2 laser light. The silver halide-based optical fibers and the chalcogenide-based optical fibers can guide infrared laser light having a longer wavelength but cannot guide visible laser light.
The silver halide-based optical fibers also have a problem of being sensitive to visible laser light having a short wavelength, resulting in silver being deposited to increase loss.
In the meantime, apart from these solid optical fibers, hollow optical fibers are used as an infrared light waveguide path. For the hollow optical fibers, a dielectric thin film coating a metal inner wall is set to have a thickness with which the reflectance is maximum for light in the wavelength region to be guided. Therefore, an appropriately designed hollow optical fiber can guide desired infrared laser light and also visible laser light.
In the medical and industrial fields, infrared laser light is used as an energy source that causes a physical or chemical change to a substance to be illuminated. Therefore, the infrared laser light is required to have a high energy output and a high guiding efficiency. By contrast, visible laser light is merely required to have a guiding efficiency with which the visible laser light is visually recognizable.
As described above, infrared laser light used in the medical and industrial fields is of a high energy output. Therefore, it is highly important for safety to guarantee that the waveguide path is sound. More specifically, for solid optical fibers, it is needed to consider the risk of end surface damage, melting, and rupture. In the case of hollow optical fibers, the risk of end surface damage is lower than in the case of solid optical fibers, but it is required to consider the risk of breakage, which may be caused by bending, melting, contamination with foreign substances, or the like.
According to an optical structure that has been put into practice as a measure for preventing infrared laser light from illuminating an unintended position as described above, abeam combiner is used on the exit side so that infrared laser light and visible light are superimposed on each other.
Specifically, visible light is guided by a conventional quarts-based optical fiber, whereas infrared laser light is guided by an infrared light waveguide path formed of a solid optical fiber or a hollow optical fiber, both of which are formed of an infrared light-transmissive material mentioned above. Namely, the optical structure in which a beam combiner is used on the exit side so that infrared laser light and visible light are superimposed on each other includes two waveguide paths. The central axis of the infrared laser light and the central axis of the visible light, both on an illumination target illuminated with the two types of laser light, are matched to each other by a beam combiner that combines the two types of light.
With the optical structure in which the invisible infrared laser light and the visible laser light are superimposed on each other on the exit side, the position illuminated with the infrared laser light can be visually recognized by visible laser light. Thus, the infrared laser light can be prevented from illuminating an unintended position. However, even in the case where the waveguide path of the infrared laser light is damaged, the visible laser light, which is guided by a waveguide path different from the waveguide path that guides the infrared light, normally exits. Therefore, the soundness of the waveguide path that guides the infrared invisible light cannot be checked.
A method for monitoring the soundness of the waveguide path is proposed by, for example, Patent Document 1. According to the method proposed by Patent Document 1, an outer circumferential surface of an optical fiber that guides laser light is covered with a conductive film. Electrical characteristics of the conductive film are changed in accordance with the state of breakage of the optical fiber. This is used to check the soundness of the optical fiber.
However, the method disclosed in Patent Document 1 requires formation of the conductive film on the outer circumferential surface of the optical fiber acting as a waveguide path and also requires a power supply device used to measure the electrical characteristics of the conductive film. This complicates the system structure and makes it difficult to construct a low-cost system.
In addition, according to the method proposed in Patent Document 1, the optical fiber used as the waveguide path is electrically conducted from an end on the light source side to an exit tip. When the optical fiber is used for an application in the medical field, the exit tip of the waveguide path approaches or even contacts a biological body. This may possibly cause electromagnetic hypersensitivity to the biological body.
Another method for monitoring the soundness of the waveguide path is proposed by, for example, Patent Document 2. According to the method proposed by Patent Document 2, laser light of a standard light amount is guided by a waveguide path, and the amount of the guided light is measured. When the measured value is lower than a threshold level, it is determined that the waveguide path has abnormality. In this case, the transmission of the laser light is stopped, so that the safety of the waveguide path is guaranteed.
The “standard light amount” is a safe light amount with which neither waveguide path nor the measurement device is broken. When the above-measured value is confirmed to be the threshold value or higher, the output of the laser light is increased to a desired level.
As described above, according to the method disclosed in Patent Document 2, the soundness of the waveguide path can be guaranteed in the case where light of an amount smaller than, or equal to, the standard amount is guided. However, in the case where laser light of a higher energy output is guided by the waveguide path, the waveguide path may be possibly broken by energy loss of the guided laser light itself.
As can be seen, with the method disclosed in Patent Document 2, even if the soundness of the waveguide path is confirmed in the case where laser light of an amount smaller than, or equal to, the standard amount is guided, the waveguide path may be broken when the output of the laser light is increased. It is not possible to monitor the soundness of the waveguide path in real time while laser light of a desired output level is caused to exit.
The above-mentioned threshold used to check the soundness when laser light of the standard light amount is guided is used with an assumption that a measured value is uniquely defined. However, the guiding efficiency of a waveguide path for laser light is changed by, for example, a bending state of the waveguide path, and thus is not necessarily a constant value. Especially in the case of a hollow optical fiber, a bending loss needs to be considered, and the threshold value for guaranteeing safety is changed by, for example, the state of installation of the waveguide path. Therefore, there may be a case where although the waveguide path is not damaged and is kept sound, the measured value is lower than the threshold value and thus the exit of the laser light is stopped. The threshold value needs to be adjusted in accordance with the state of use. Since such adjustment increases troublesome work imposed on a user, it is difficult for the user to adjust the threshold value in accordance with the state of use.